There are a number of situations, in the completion of oil and gas wells, where it is desirable to isolate one section of a subterranean well from another. For example, in U.S. Pat. No. 5,924,696, there is disclosed an isolation tool used alone or in combination with a packer to isolate a lower section of a production string from an upper section. This tool incorporates a pair of oppositely facing frangible or rupturable discs or half domes which isolate the well below the discs from pressure operations above the discs and which isolate the tubing string from well bore pressure. When it is desired to provide communication across the tool, the upper disc is ruptured by dropping a go-devil into the well from the surface or well head which falls into the well and, upon impact, fractures the upwardly convex ceramic disc. The momentum of the go-devil normally also ruptures the lower disc but the lower disc may be broken by application of pressure from above, after the upper disc is broken, because the lower disc is concave upwardly and thereby relatively weak against applied pressure from above.
An important development in natural gas production in recent decades has been the drilling of horizontal sections through zones that have previously been considered uneconomically tight or which are shales. By fracing the horizontal sections of the well, considerable production is obtained from zones which were previously uneconomical. For some years, the fastest growing segment of gas production in the United States has been from shales or very silty zones that previously have not been considered economic. The current areas of increasing activity include the Barnett Shale, the Haynesville Shale, the Fayetteville Shale, the Marcellus Shale, the Eagle Ford Shale in the United States, the Horn River Basin of Canada and other shaley formations in North America and Europe.
It is no exaggeration to say that the future of natural gas production in the continental United States is from these heretofore uneconomically tight gas bearing formations. In addition, there are many areas of the world where oil and gas is produced and costs are, from the perspective of a United States operator, exorbitantly high. These areas currently include offshore Africa, the Middle East, the North Sea and deep water parts of the Gulf of Mexico. Accordingly, a development that allows well completions at overall lower costs is important in many areas of the world and in many different situations.
Disclosures of interest relative to this invention are found in U.S. Pat. Nos. 7,044,230; 7,210,533 and 7,350,582 and U.S. Printed Patent Applications S.N. 20070074873; 20080271898 and 20090056955.